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Weight ratio to orbital speed Figure 4.30. Engine thrust-to-weight ratio decreases with weight ratio.

to flight duplicated engine entrance conditions of Mach 8, the maximum airbreath-ing speed for SERJ. In those test, the flight weight engine would have had an installed thrust-to-weight ratio of 22, had it been installed in an aircraft. From Figure 3.3, the mass ratio for an airbreathing speed of Mach 8 is 5. From Figure 4.30 the range of values for a weight ratio of 5 is 25 to 27. So the SERJ engine would have had a weight just slightly greater than the assumed all-rocket engine weight. This is a simple approach to estimate the operational weight of an arbitrary propulsion system. However, a word of caution: this approach is to estimate the installed engine thrust-to-weight ratio for an integrated propulsion system. It will not estimate the weight of the engine airbreather approach shown in Figure 2.14, as that is an impracticable system by any standard. It is very easy to have estimates that destroy an airbreathing approach in that, to some, they appear perfectly reasonable when they are in fact based on misinformation. The relationship given in equation (4.16) will give an obtainable value, given the industrial capability available today and the history of actual integrated airbreathing cycles.

Figure 4.30 shows that air augmented rockets and ram rockets have lower engine thrust-to-weight ratios because of the secondary air duct weight. ACES has a lower engine thrust-to-weight ratio because of the weight of the air separation hardware. And, as postulated, PDEs have a higher engine thrust-to-weight ratio because the pumping hardware is lighter than the conventional rocket turbopumps, with a lower required launcher takeoff thrust to weight ratio. One of the advantages of wing-supported horizontal takeoff is an acceptable lower engine thrust-to-weight ratio. So as discussed earlier in conjunction with Figure 3.24, if the mass ratio permits horizontal takeoff without serious weight penalty, it has the operational advantage to

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